Various entities build Stirling coolers equipped with a fluid bearing linked to compliant structures, such as alternator rods. This design is used for centering the reciprocating compressor piston inside the compressor bore during normal operation.
U.S. Pat. No. 5,525,845 describes a “compliant” linkage for mechanical transducers having a fluid bearing-supported, reciprocating body in a chamber. This reciprocating body can be a compressor piston used for a linear Stirling cooler. The compliant linkage allows the piston to conduct the required lateral movements for proper piston-to-compressor bore alignment. FIGS. 1A-B show the alignment process generated by the gas bearing.
The piston is typically connected to a leaf spring. The axial spring stiffness of a leaf spring is relatively low and the radial stiffness is typically high. The leaf spring also allows the piston axis to be rotated and aligned with little required torque for an axis-parallel orientation relative to the symmetry axis of the compressor bore. However, the rotation of the piston axis alone is not sufficient for proper piston-to-compressor bore alignment. A second, lateral movement of the piston axis is necessary to accomplish the alignment as shown on the right side of FIG. 1A. This design was to connect the leaf spring with a compliant structure or linkage, which works like a lateral spring with a relatively low spring stiffness, thus high lateral compliance “ . . . sufficient for the centering forces exerted by the fluid bearing to at least equal the sum of all other lateral forces exerted on the body” (the piston) “including the lateral force exerted upon the body by the linkage, during normal operation of the transducer”, e.g. the compressor portion of a Stirling cooler.
The compliant structure can be realized, for example, by using laterally flexible and axially stiff “Alternator Rods.” This configuration is schematically shown in FIG. 1A, which also gives an alternative option for a compliant structure. A modified leaf spring, schematically shown in FIG. 1B, can also function as a laterally compliant component.
There are a number of disadvantages to the prior designs described herein. The assembly of the piston and other related components require several operator dependent manufacturing processes, which are critical to ensure cooler performance and long lifetime. Many cooler production problems are related to improper piston and displacer alignment.
U.S. Pat. No. 5,525,845 points out the gas bearing has to be at least equal to the sum of all other lateral forces.
This means that the piston has to be properly pre-aligned during the assembly process for proper functionality as a non-friction bearing. Deformed or misaligned alternator rods can cause additional lateral forces, larger than the provided gas bearing forces, which are limited by the maximum available gas bearing pressure.
Piston alignment problems or additional piston side forces can be even more critical, for example in the case where a Stirling cooler is running at minimum input power condition and the gas bearing stiffness reaches a minimum as well. The gas bearing stiffness is a function of the generated input power-dependent pressure wave inside the compression space of the cooler.
The quality of the pre-alignment process is also determined by the quality and thus the tolerances of the piece parts. Particularly tight tolerances—a few thousands of an inch to a few ten-thousandths of an inch—have to be maintained to minimize the introduced piston side forces during manufacturing.
The assembly process in production has to be conducted with care, preferably by trained operators. Tools are helpful. However, alignment process quality is still operator-dependent.
Alternative methods use complex and expensive methods for aligning the gas bearing. For example, U.S. Pat. No. 7,043,835 provides a computer system for sensing the location of a body within a bore and using microactuators to adjust the position of the body to center it within the bore.
The following references are cited as being of potential background interest: U.S. Pat. No. 5,525,845, issued Jun. 11, 1996, entitled: Fluid Bearing With Compliant Linkage For centering Reciprocating Bodies, (Beale et al.), U.S. Pat. No. 2,907,304, issued Oct. 6, 1959, entitled: Fluid Actuated Mechanism, (Macks), U.S. Pat. No. 4,545,738, issued Oct. 8, 1985, entitled: Linear Motor Compressor With Clearance Seals And Gas Bearings, (Young), U.S. Pat. No. 4,387,568, filed Jun. 14, 1983, entitled: Stirling Engine Displacer Gas Bearing, (Dineen), ICC 11 Paper: Performance and Reliability Improvements in a Low-Cost Stirling Cryocooler, (Hanes), U.S. Pat. No. 7,137,259, entitled: Cryocooler Cold-end Assembly Apparatus And Method, (O'Baid et al.). The foregoing references are hereby incorporated by reference as if fully set forth herein.